The demands on automobiles for better fuel economy have been increasing in recent years as concern with environmental issues has been rising. Good fuel economy is also being required of the rubber compositions used for automotive tires. For example, rubber compositions containing a conjugated diene polymer (e.g., polybutadiene, butadiene-styrene copolymer) and a filler (e.g., carbon black, silica) are used for the rubber compositions for automotive tires.
Patent Literature 1 and Patent Literature 2, for example, propose methods for enhancing the fuel economy. The method of Patent Literature 1 uses a diene rubber that has been modified with an organosilicon compound containing an amino group and an alkoxy group. The method of Patent Literature 2 uses a specific silane coupling agent containing a mercapto group. These days, however, further enhancement of the fuel economy has been demanded.
Other methods for enhancing fuel economy are known, such as reduction of the amount of a reinforcing filler or use of a silane coupling agent containing a mercapto group. In the former method, however, the hardness of the rubber composition is reduced such that a tire containing the rubber composition softens. Thus, the handling property (handling stability) or wet-grip performance of vehicles may be reduced. Also, the reinforcing property is not sufficiently achieved so that the abrasion resistance tends to decrease. The latter method tends to reduce the scorch time to deteriorate, the processability.
Normal rubber compositions for tires contain oils like aromatic oil to enhance the processability or wet-grip performance. However, addition of such oils tends to increase the rolling resistance or reduce the fuel economy.
As mentioned earlier, fuel economy assumes an inverse relationship with such properties as wet-grip performance or abrasion resistance. Thus, it has been difficult to obtain these properties at high levels in a balanced manner.